1. Field of the Invention
The present invention relates to an antenna device used to transmit and receive a radio signal, and particularly to an antenna device formed by simple combination of planar conductors including a radiating conductor and a ground conductor plate disposed to face each other with an insulating material interposed therebetween.
2. Description of the Related Art
In wireless communication using a radio wave communication method, a signal is transmitted with the use of a radiation field generated upon passage of current through an aerial (an antenna). The antenna has a variety of types. An antenna having a wide band characteristic can be used in communication which transmits and receives signals by diffusing the signals over an ultra wide frequency band such as a UWB (Ultra Wide Band). Further, a small-size antenna contributes to a reduction in size and weight of a wireless device.
In particular, an antenna configuration satisfying a request for a thinner antenna includes an antenna device configured such that a radiating conductor and a ground conductor plate are disposed to face each other with an insulating material interposed therebetween, i.e., a microstrip patch antenna (hereinafter abbreviated simply as the patch antenna). The shape of the radiating conductor is not particularly determined, but is rectangular or circular in most cases. The thickness of the insulating material interposed between the radiating conductor and the ground conductor plate is generally set to be equal to or less than one tenth of the wavelength of a radio frequency. Thus, the patch antenna can be formed into a substantially thin shape. Further, the patch antenna can be manufactured by an etching process performed on an insulating material substrate copper-clad on both sides thereof, and thus can be manufactured with relative ease. That is, it is relatively easy to manufacture the patch antenna.
For example, a magnetic microstrip patch antenna has been proposed in which short-circuiting conductor plates for making a radiating conductor and a ground conductor conductive are appropriately disposed at respective positions for suppressing excitation in an undesired mode, to thereby suppress disturbance in a radiation pattern at an end of a band, and in which a magnetic material having a relative permittivity of one or higher and having a multilayer structure including alternate lamination of a magnetic layer and an air layer is used to fill the gap between the radiating conductor plate and the ground conductor plate, to thereby realize unidirectivity in a wide bandwidth (see US Patent Application No. 2005/253756, for example).
A normal printed board has a structure in which a thin dielectric plate is vertically sandwiched by two conductor plates. If the printed board is structured such that the lower conductor plate is used as a ground (GND), and that the upper conductor plate is formed into a rectangular or circular shape and fed with electric power, a patch antenna can be formed and easily integrated with the circuit board.
FIGS. 15 and 16 illustrate an exemplary configuration of a patch antenna formed on the printed board (FIG. 15 is a top view of the printed board and FIG. 16 is an oblique view of the printed board). The patch antenna illustrated in FIGS. 15 and 16 is designed with an antenna formed by an upper conductor plate (101, 201) (a radiating element) that is regarded as a resonator. Further, current flowing along an end edge of the conductor plate is considered to be equal to current flowing through a parallel transmission line 104 extending across the dielectric material (102, 202). Therefore, the patch antenna has a wavelength reduction effect according to the relative permittivity of the dielectric material (202). If it is assumed that a length L of the radiating element is equal to a width W of the radiating element, the patch antenna is represented by the following equation. FIG. 16 also shows a lower conductor layer 203 that is used as a ground (GND). FIG. 15 shows a connection point 103 between parallel transmission line 104 and the upper conductor plate 101.
                    Formula        ⁢                                  ⁢        1                                                            L        =                  W          =                                    λ                              2                ⁢                                                      ɛ                    eff                                                                        =                          λ              2                                                          (        1        )            
Herein, ∈eff represents the effective permittivity of the dielectric substrate, and λg represents the effective wavelength. The effective permittivity ∈eff can be determined on the basis of the permittivity and the thickness of the dielectric substrate and the value of the width W of the antenna (=the length L of the antenna). The above Equation (1) shows that, if the length or width of the antenna (the radiating element) is reduced to half the effective wavelength, resonance occurs to radiate radio waves of a resonance frequency.
Communication systems of recent years can be divided into narrow band communication and wide band communication. Frequency components which can be radiated by the patch antenna include a frequency f determined by the following Equation (2) on the basis of the effective wavelength λg and a higher harmonic component thereof.
                    Formula        ⁢                                  ⁢        2                                                            f        =                  c                      λ            g                                              (        2        )            
That is, the patch antenna generally tends to operate in a narrow band, and thus is considered to be unsuitable for, for example, a PAN (Personal Area Network) system, the operable band of which is necessary to be wide. Bandwidths having a VSWR (Voltage Standing Wave Ratio) of two or less are generally on the order of a few percent, depending on a design parameter. Due to this disadvantage, there is an issue that it is difficult to use the patch antenna in the wide band communication.
A planar patch antenna including a ground on the back surface thereof on a dielectric multilayer board has a narrow band. To ensure the wide band characteristic in the patch antenna of the related art, therefore, a structure not including the ground on the back surface of the antenna is generally employed. In such a case, however, the structure of a housing of an electronic device is complicated in design.
Further, in many of wireless communication techniques in the past, which assume long-distance communication, it suffices if only the behavior of the antenna in a far field is taken into account. In recent years, however, there have been increasing cases assuming close-range communication. Thus, it has been becoming necessary to understand phenomena occurring in a near field of the antenna, in which the communication distance is equal to or shorter than the wavelength.